Radiation therapy for cancer treatment has been in use for several decades. Modern radiation therapy systems typically generate high intensity x-rays by bombarding a suitable target with high energy electrons. X-rays are emitted from the target in a generally conical pattern and are initially confined to a generally rectangular beam by moveable, x-ray blocking “jaws” in the head of the system. Typically, the patient is positioned about 1 meter from the x-ray target and, when fully open, the jaws define a square treatment area that is about 40 cm×40 cm at the patient plane. Rarely, however, can the system jaws alone be used implement a suitable treatment plan. While high energy x-rays are used most commonly in radiotherapy, other types of radiation, including electron and proton beams are also known in the art. The nature of the radiation beam can have significant impact on the appropriate shielding and field shaping collimator designs, particularly for those particle beams that are not subject to an exponential attenuation but rather are stopped by successive interactions that cause energy loss. Application of the present invention, however, is not dependent upon use of any particular type of radiation beam, thus any type-specific descriptions are for exemplary purposes only.
It is usually desirable to irradiate only a precisely defined area or volume conforming to a tumor, and to irradiate the target site from multiple angles. Multi-leaf collimators (MLCs), such as described in the co-assigned U.S. Pat. No. 4,868,843, issued to Nunan (the disclosure of which is incorporated by reference), have been almost universally adopted to facilitate shaping of the radiation beam so that the beam conforms more closely to the site being treated, i.e., the beam is shaped to conform to the shape of the tumor from the angle of irradiation. Subsequent to its introduction, the MLC has also been used to perform a technique known as “Intensity Modulated Radiotherapy” (IMRT), which allows control over the radiation doses delivered to specific portions of the site being treated. In particular, IMRT allows the intensity distribution of the radiation reaching the patient to have almost any arbitrary distribution. MLCs are also used, for example, in arc therapy; wherein the gantry system, and hence the radiation beam, is moved along an arc while the patient is being irradiated.
The basic operation of an MLC is well known in the art and will not be described in detail. In summary, the MLC comprises moveable leaves that are positioned to create an opening or aperture that can be viewed as lying in a plane that is perpendicular to the general direction of radiation. The MLC aperture can have any arbitrary shape within the mechanical limits of the device, through which the radiation is delivered. Static IMRT (s-IMRT) can be implemented by iteratively positioning the leaves of an MLC to provide the desired field shapes. Superposition of the individual apertures, which typically number from one to hundreds, collectively delivers the desired dose distribution. This approach is static in the sense that the leaves do not move when the beam is on.
Alternatively, in systems such as those sold by the assignee of the present invention, a dynamic IMRT (d-IMRT) method can be implemented using, for example, a “sliding window” approach, in which the leaves of an MLC are moved continuously across the beam when the beam is on. Each leaf pair (44a, 44b in FIG. 4) creates a time-dependent opening that traverses the target area from one side to the other, creating the desired fluence pattern. By adjusting the speed of leaf motion and the separation of the leaves, different portions of the treatment field can be irradiated with different doses of radiation as prescribed in a treatment plan.
Although the leaves are positioned closely together, many MLCs have a problem with inter-leaf leakage, where radiation leaks through the space between the leaves. This leakage causes striping in the final delivered fluence, which is undesirable because it causes a variance between the delivered dose and the dose prescribed under the treatment plan. To minimize this inter-leaf leakage, some MLC models use leaves 10a with a tongue 12a and groove 14a design (see FIG. 1A). The tongue of each leaf fits into the groove of the adjacent leaf, blocking radiation from leaking between the leaves and, thereby, reducing striping in the final fluence delivery. Unfortunately, the tongue and groove design can also have the opposite effect; increasing the stripe patterns in the final fluence. This increase occurs when a tongue of one leaf is exposed, i.e., not positioned within the groove of another leaf, during delivery of radiation. The exposed tongue partially blocks radiation from passing, which causes a stripe in the final fluence.
Throughout this disclosure, the “tongue and groove effect” specifically refers to the negative effect in IMRT delivery that is caused by a thickness of leaf material on the side of a leaf exposed to the incident rays of the radiation beam that is less than the full thickness of the leaf. This thickness can be in the form of an exposed tongue, such as 12a in FIG. 1A, or an exposed groove, such as 14a, but is not limited to this type of leaf design. An “exposed” groove can be thought of as missing material to make room for an adjacent tongue with the remaining leaf material exposed to rays of the beam alone. Exposure of either a tongue or a groove can cause striping in the IMRT delivery. Depending on the geometry of the MLC, the striping caused by an exposed groove may be less significant than that caused by an exposed tongue (such as the leaf design in FIG. 1A), although in most current designs the effect from an exposed tongue is very similar to, if not the same as, the effect from an exposed groove.
The tongue and groove pattern can be at any height along the leaf side and need not be in the form of a centrally located groove 14a as shown in FIG. 1A. Some MLC designs employ a single stepped side wherein the tongue is a protrusion in one half of the leaf while the “groove” is a mating inset. Other MLC designs use a more symmetrical, slanted leaf pattern (e.g., 10b in FIG. 1B), where the effects of an exposed tongue 12b and a corresponding exposed groove 14b are of equal magnitude. In such a design, the exposed tongue and groove may simply be surfaces that protrude or recede with respect to the diverging beam rays by virtue of a purposeful difference between the divergence of the beam rays and the side of the leaf. Other leaf designs may be employed to reduce the effect of inter-leaf leakage that introduces a tongue and groove effect in IMRT. The present invention described below is applicable to any of these designs because although a tongue or groove feature may not be evident in the leaf design, the tongue and groove effect may nonetheless be observed in a resulting IMRT field.
While the tongue and groove effect may be used advantageously in certain advanced optimization techniques, such as in s-IMRT pattern calculation or d-IMRT minimum-leaf-gap-effect optimization (which may also be used in combination with the present method by limiting the area to which the method is applied), the tongue and groove effect generally has a negative impact on the quality of the delivery. In particular, current IMRT techniques at most evaluate the effect, but do not take it into account in leaf position calculations.